What Is It?
Depression is more than just a passing blue mood, a "bad day," or temporary sadness. The most common symptom is a low mood that can sometimes appear as irritability. Often the person with depression is not be able to enjoy activities that he or she normally enjoys.
With major depression, there is a profound sadness or a sense of despair. The symptoms of major depression are defined as lasting at least two weeks, but usually they go on much longer.